The present invention relates to an apparatus and method for balancing the individual channel currents in a multi-phase DC/DC converter.
There are no known prior art devices or methods that specifically balance the channel currents of multi-phase converters.
Personal computers have direct current (DC) power supplies to regulate their operating voltage and current. Early personal computers operated their circuits at xc2x15 volts and drew several amps of current. In order to speed-up performance, operating voltages were dropped to the range of xc2x11.5 to 1.0 volts and currents have risen to 50 or more amps. It is more economical to provide the 50 or more amps from several power sources rather than from a single source. This has led many power supply manufacturers to provide multi-phase converters with two or more current channels. While there are more component parts in multi-phase systems, the parts themselves are smaller and typically less expensive than the high-power parts which must be used in a single-converter having similar current capabilities.
When multi-phase converters supply the same load there is often a voltage mismatch between the channels. If two or more channels have even slightly different output voltages, current will flow mostly from the channels with the highest voltage. Some converters have the ability to sink as well as source output current. In those converters, current may flow from one channel to another, regardless of load current. This can lead to excessive power dissipation. Additionally, the load that these converters supply must be limited below the combined full load capability of the individual channel.
Without the capability to share the load current, each converter channel provides a current proportional to the average phase voltage and the net converter resistance. The average phase voltage is approximated by:
VPH=(VINxe2x88x92VUP)xc2x7Dxe2x88x92VLOW(1xe2x88x92D) 
where:
VIN is the input voltage,
VUP is the voltage drop across the upper switch,
VLOW is the voltage across the lower switch, and
D is the duty cycle.
The net converter resistance includes the summation of the inductor winding resistance, any trace resistance, and the time multiplexed resistance of the upper and lower power switches.
In multi-phase converters, the ability to equally share the load current depends upon the matching of parameters and components between each of the phases or channels. Current sharing is particularly sensitive to any duty cycle mismatch between channels. Matching the duty cycle of multiple phases is difficult because of inherent component mismatches that can induce timing errors. As a result, any channel may be forced to carry significantly more than its proportional share of the load current. For example, in a four-phase converter with four converter channels, one channel may carry 40% of the load current while the other channels each carry 20%, rather than each channel carrying the ideal 25%. Thus, each channel must be sized to carry at least 40% of the projected output current, or 15% more than its proportionate share. Designing each of the four channels for 40% of the projected output current, rather than for 25% of the projected output current, requires the use of oversized power output transistors and passive components, such as, for example, inductors and resistors, in order for each channel to safely conduct a higher proportion of load current. Since the distribution of the load varies, each power transistor must be larger than needed for the total load. However, if the load is more evenly distributed smaller transistors as well as smaller passive components can be used to achieve the same load current capability as oversized prior art systems. Smaller transistor and passive components are less expensive and more efficient than larger, higher-power components.
Therefore, what is needed in the art is a multi-phase converter which equally shares the load current between each of the phases or channels.
Furthermore, what is needed in the art is a multi-phase converter which uses smaller transistors and smaller passive components to produce a given load current capability, thereby making it less expensive to produce and sell.
The present invention provides an apparatus and method for balancing the channel currents in a multi-phase DC/DC converter.
The invention comprises, in one form thereof, a multi-phase DC/DC converter having an output voltage and including a plurality of converter channels. Each converter channel includes a converter channel input and a converter channel output. Each converter channel is configured for generating a converter channel current and for adjusting said converter channel current in response to a control signal electrically connected to each converter channel input. A control circuit generates an error signal representative of a comparison of the converter output voltage to a reference voltage. The control circuit includes a plurality of control circuit channels, each of which correspond to a converter channel. Each control circuit channel generates a channel current signal representative of a corresponding converter channel current, and generates a differential channel current signal representative of a comparison of the channel current signal to an average current signal. The average current signal is representative of an overall average current for the converter channels. Each control circuit channel generates a differential error signal representative of a comparison of the error signal to the differential channel current signal. Each control circuit channel includes a pulse width modulator having a ramp input and a control input. The control input is electrically connected to the differential error signal. The pulse width modulator generates the control signal based upon the differential error signal. The control signal is electrically coupled to a corresponding converter channel input. The control circuit generates the average current signal.